1. Field of the Invention
This invention relates generally to the completion of gas wells and more particularly to a method of predicting the on-set of solids production in high flow rate gas wells.
2. Description of the Related Art
High-rate gas well completions are common practice in offshore developments and among some of the most prolific gas fields in the world. These fields typically have reservoirs that are highly porous and permeable but weakly consolidated or cemented, and sand production is a major concern. Because of the high gas velocity in the production tubing, any sand production associated with this high velocity can be extremely detrimental to the integrity of surface and downhole equipment and pose extreme safety hazards. Prediction of a maximum sand free production rate is therefore critical, not only from a safety point of view but also economically. The unnecessary application of sand control techniques, as a precaution against anticipated sand production, can cause an increase in completion costs and a possible reduction in well productivity. However, if operating conditions dictate the need for sand exclusion, such techniques can make a well, which otherwise could have been abandoned or not developed, extremely profitable.
As gas flows through a perforation cavity or through a horizontal borehole, the gas pressure in the flow passage is less than the gas pressure in the formation pores. The greater the difference between the two pressures, the higher the flow rate. This difference is called the drawdown pressure. Two mechanisms responsible for sand production are compressive and tensile failures of the formation surrounding the perforation cavity or horizontal borehole. Compressive failure refers to tangential stresses near the cavity wall exceeding the compressive strength of the formation. Both stress concentration and fluid (liquid or gas) withdrawal can trigger this condition. Tensile failure refers to tensile stress triggered exclusively by drawdown pressure exceeding the tensile failure criterion. Tensile failures predominate in unconsolidated sands and compressive failures in consolidated sandstone. The near borehole stresses cause desegregation of the formation while the fluid drag forces provide the medium to remove the failed materials. The drawdown pressure at which the formation begins to fail and produce sand is called the critical drawdown pressure (CDP). The ability to accurately predict CDP is critical to optimizing the well completion strategy.
For CDPs in gas wells, an analytical spherical cavity stability model that considers the pressure dependent density for a non-ideal gas has been proposed: see Weingarten, J. S., and Perkins, T. K.: “Prediction of Sand Production in Gas Wells: Method and Gulf of Mexico Case Studies”, paper SPE 24797 presented at the 67th Annual Technical Conference and Exhibition, Oct. 4–7, 1992. This model assumes a steady state Darcy's seepage force with the Mohr-Coulomb yield criterion to establish the pressure gradient near the cavity face. The maximum permissible, or critical, drawdown is arrived at by limiting the net tensile stresses at the cavity wall to zero. Because this tensile model assumes only Darcy's flow regime, its use is limited to low-rate gas well applications. One of the characteristics of a high gas-rate flow in the reservoir is the deviation from Darcy flow in describing the pressure gradients over the whole range of fluid interstitial velocity. This is especially true in a limited region around the wellbore where the pressure drawdown is high and the gas velocity can become so large that, in addition to the viscous force component represented by Darcy's law, there is also an additional force due to the acceleration and deceleration of the gas particles, referred to as the non-Darcy component.
Another approach proposed a cavity stability predictive model that incorporates the effects of non-Darcy flow for a cylindrical perforation tunnel: see Wang, Z., Peden, J. M., and Damasena, E. S. H.: “The Prediction of Operating Conditions to Constrain Sand Production from Gas Well”, paper SPE 21681 presented at the Production Operations Symposium, Apr. 7–9, 1991. The analytical model uses a gas flow model to calculate the pore pressure distribution associated with various production conditions, while a stress model with pore pressure input evaluated from the gas flow model is used for the determination of the stress and strain distributions. The stability of a perforation is assessed when the equivalent plastic strain has reached a certain critical value. The results from this non-coupled, compressive failure model suggest that non-Darcy flow has far more effect on the perforation cavity instability than Darcy flow, particularly in the case of weakly consolidated rocks.